Are you deleting a page when you're saving? That's what the trace indicates is happening.

When you delete a page, Exhibit Builder moves all the children of the page that's getting deleted up a level. That looks like what's causing the slug conflict: one of the child pages getting moved has the same slug as one of the pages that already exists on the higher level.

Unless I'm mistaken, in the video you posted, you clicked the "x" button for a page, and saved that "change." Doing that is the same as clicking the big Delete button on the edit page for that specific exhibit page. The big Delete button visible in the video you posted is the one for the Exhibit, and it deletes the entire exhibit and all its pages.

As for moving forward, I think changing the slugs of the pages you don't want anymore through the interface is the easier option. You could do it through SQL, but you'd also have to go and reassign the child pages, the ones you do want, to have the right parent page IDs, and it's just slightly more complicated.

If you just go in one pass and edit the slugs for those top pages (assuming its the top ones you don't want) and give them slugs like "junk1," "junk2," "junk3," etc., then you'll be able to go right back and delete them all without Omeka complaining about conflicting slugs.